Ruby from the Sanbagawa metamorphic belt, in the Hodono Valley, Ehime Prefecture, Japan

Abstract

Ruby, purplish red to pink, found in the Hodono valley, Ehime Prefecture, occurs in an amphibole-zoisite rock with a minor amounts of garnet, paragonite, and chlorite.
The corundum is idiomorphic granular, up to 5 cm in diameter, and transparent purplish red to colorless. Optical properties are uniaxial (−) ε=1.761(2), and ω=1.769(2). Density (meas.) is 3.887(5). Red fluorescence is displayed by long (3650 Å) and short (2536 Å) wave ultraviolet rays. Cell dimensions are a=4.760(1), c=12.992(3)Å, c⁄a=2.729, and cell volume 254.9(1)ų. Infrared absorption bands occur at 780 cm⁻¹, 641 cm⁻¹, 595 cm⁻¹, 450 cm⁻¹, and 378 cm⁻¹.
Microprobe analyses for the minor oxide abundance of corundums with dark purplish red to light purplish pink color are Fe₂O₃ 0.44−0.39 vrt%, Cr₂O₃ 0.44−0.01 wt%, SiO₂ 0.0−30.00 wt%, and MnO 0.01−0.00 wt%. The abundance of Cr₂O₃ varies among different grains or within a single grain, and corresponds to the shade of red.
The mineral assemblage of the corundum is briefly discussed.